Atom Transport in Lithium Doped Magnesium Fluoride Crystals.

From a previous study of Li doped MgF(,2), lithium, at low temperature, is known to form a dumbell pair centered on a Mg site. This pair can be thought of as the association of a substitutional Li(,S) with an interstitial Li(,I). We have investigated the nature of the Li defect and the corresponding transport properties of doped MgF(,2) at intermediate and high temperatures. At intermediate temperatures we have measured electrical. conductivity, primarily // to the c axis, as a function of lithium. concentration and have found (sigma)// greatly enhanced. The large. conductivity is attributed to Li(,I) created by the dissociation of pairs. We have identified two separate temperature regions in the (sigma)// conductivity. In the high temperature region (300 to 600(DEGREES)C) we have found (sigma) (PROPORTIONAL) C(,Li) in accordance with an almost complete dissociation model. At high enough concentrations (>(, )1000ppm), (sigma) departs from a linear dependence to approach a C(,Li)(' 1/2) dependence interpreted in terms of an almost complete association model. The Arrhenius energies in the high temperature (sigma) region fall between 0.8 and 0.9 eV. In the low temperature region, we observe a supralinear dependence, (sigma) (PROPORTIONAL) C(,Li )and higher Arrhenius energies (> 1 eV). The conductivity may then be controlled by the dissociation of Li(,I) from pair clusters. The conductivity I to the c axis is also enhanced by lithium. (sigma) I is attributed to fluorine interstitials F(,i) compensating for an excess of Li(,I) over Li(,S). Above 650(DEGREES)C we have measured the Li diffusivity D(,Li) by means of the chemical diffusion of lithium in and out of MgF2 crystals. D(,Li) is abnormally fast reaching 10('-7) cm('2)/sec at 1000(DEGREES)C. Such a fast diffusion is attributed to the presence of a large fraction of Li(,I). The small anisotropy ratio, D(,//)/D(,I) (TURN)3, for Li diffusion shows, however, that Li cannot be rate controlling. This is a case of ambipolar diffusion controlled by F(,i). Below 800(DEGREES)C we observe a sharp drop in the diffusivity curves for both D(,//) and D(,I). We interpret this drop as a region in which Li defects, mostly Li(,I) above 800(DEGREES)C, revert to a mostly self-compensated mode of solution (i.e. Li(,I) + Li(,S)). The presence of a large excess of Li(,I) over Li(,S), at high. temperatures, is exemplified by the greatly enhanced diffusivity. of Mn in MgF(,2), in the presence of lithium. Such an enhancement. requires the presence of magnesium vacancies, above their. intrinsic concentration level, which can only be introduced by Li(,I). *Technical Report under National Science Foundation Grants DMR 77-07141 and 80-06326.